Apparatus for measuring characteristics of an optical fiber

ABSTRACT

The present invention aims to provide apparatus to measure the beat length, the correlation length, the polarization mode dispersion and other characteristics of single mode optical fibers related to the two polarization modes of the fiber at different positions along the length of the fiber. The present invention also aims to provide apparatus for the measurement of characteristics of an optical fiber with access to only one end of the fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of PCT/IB98/00225 filedFeb. 10, 1998 under the International Convention.

FIELD OF THE INVENTION

This invention relates to an apparatus for measuring characteristics ofan optical fiber. More specifically, this invention relates to apparatusfor measuring characteristics of an optical fiber such as beat length,correlation length, and polarization mode dispersion, at differentpositions along the length of the optical fiber.

BACKGROUND OF THE INVENTION

Within the field of optical fiber telecommunications, the current upperlimit on the bit rate of detection arises from the birefringencedistributed along the length of a single-mode optical fiber. Thebirefringence may be due to non-circularity of the core of the opticalfiber, and to stresses within the fiber. The birefringence may also varyalong the length of the fiber. However, the birefringence may also becaused by external forces acting upon the fiber, and to temperaturevariations, and these effects vary both along the length of the fiberand with time. The overall birefringence of a length of fiber thusvaries over time with a magnitude which is random.

An ideal single-mode optical fiber guides optical power in thefundamental mode as two identical but orthogonal polarization modes, sothat the modes are completely interchangeable. However, imperfections inthe fiber and the effect of external parameters lead to the opticalpower within the two polarization modes both differing in magnitude andtravelling at slightly different speeds so that a differential groupdelay exists between the modes. The birefringence determines themagnitude of the differential group delay, and the optical power withinthe two polarization, and so the width of an optical pulse travellingalong the optical fiber will vary randomly by an amount determined bythe random fluctuations in the birefringence. This effect is called thepolarization mode dispersion, and it is of particular concern because itlimits the performance of optical fiber telecommunications systems in away that cannot be predicted accurately, and hence cannot becompensated.

Although polarization mode dispersion is usually considered to be acharacteristic of the total length of an optical fiber, the effectswhich give rise to it may act over relatively short sections of thefiber. In particular, the polarization mode dispersion of an opticalfiber cable may increase significantly after installation, arising froma change in the birefringence over one particular short length withinthe fiber. Accordingly, it would be useful to be able to measurecharacteristics related to the two polarization modes of a single-modeoptical fiber, such as polarization mode dispersion, at differentpositions along the length of the optical fiber so that any local effectcan be identified at a particular length position in the optical fiber.This would be particularly useful if the measurement were able to bemade with access to just one end of the fiber in the same way thatoptical time domain reflectometers are used to measure the optical lossof fibers. Existing commercial apparatus can measure the overallpolarization mode dispersion of an optical fiber and requires access toboth ends of the fiber. The existing commercial apparatus does notenable the measurement of the polarization mode dispersion at differentpositions along the length of the optical fiber. The magnitude of thebirefringence as it varies along the fiber may be characterized as abeat length, and the statistical correlation between two sections offiber may be related by a correlation length. Both these parameters areuseful for describing the behavior of the fiber, and the environment itis experiencing, and are inherently characteristics of length positionwithin the fiber.

There are a number of known methods of measuring polarization modedispersion, and associated characteristics of single-mode opticalfibers, which provide a single measurement for the total lengthrequiring access to both ends of the fiber. In addition, optical timedomain reflectometry is a well-established technique for measuring theoptical loss of an optical fiber at different positions along the lengthof the fiber and requiring access to only one end of the fiber. Thepresent invention is based on the discovery that it is possible to applythe existing measurement techniques of polarization mode dispersion tomodified versions of optical time domain reflectometry apparatus, andthus to derive useful measurements.

OBJECTS OF THE INVENTION

The present invention aims to provide apparatus to measure the beatlength, the correlation length, the polarization mode dispersion, andother characteristics of single mode optical fibers related to the twopolarization modes of the fiber at different positions along the lengthof the fiber. The present invention also aims to provide apparatus forthe measurement of characteristics of an optical fiber with access toonly one end of the fiber.

SUMMARY OF THE INVENTION

According to a non-limiting embodiment of the present invention, thereis provided apparatus for measuring characteristics of an optical fiber,at different positions along the length of the optical fiber, whichapparatus comprises:

tunable source means for providing optical pulses of light which have avariable wavelength and a narrow wavelength bandwidth;

polarization selecting coupler means which comprises an input port, abi-directional port and an output port, and which is for conveying lightbetween the input port, the bi-directional port and the output port,such that a state of polarization of the optical pulses of light inputat the input port becomes a particular launch state of polarization oflight output from the bi-directional port, and such that one or moreparticular receive states of polarization of light input at thebi-directional port become one or more separate channels of light outputfrom the output port;

optical connector means for making an optical connection between thebi-directional port of the polarization selecting coupler means and oneend of the optical fiber, so that the light output from thebi-directional port of the polarization selecting coupler means islaunched into the optical fiber and light backscattered within theoptical fiber is received as the light input at the bi-directional portof the polarization selecting coupler means;

photodetector means for converting the intensity of each of the one ormore separate channels of light output from the output port of thepolarization selecting coupler means into one or more separateelectrical signals;

launch controller means for controlling the timing, duration andwavelength of the optical pulses provided by the tunable source means,and for specifying the particular launch state of polarization of thelight output from the bi-directional port of the polarization selectingcoupler means;

receive controller means for controlling the timing of measuring theelectrical signals provided by the photodetector means, and forspecifying the one or more particular receive states of polarization oflight input at the bi-directional port of the polarization selectingcoupler means; and

processor means for measuring and processing the electrical signalsprovided by the photodetector means into measurements of thecharacteristics of the optical fiber.

The tunable source means may be a semiconductor laser diode, a solidstate laser, or a gas laser, with a wavelength bandwidth preferably lessthan 0.1 nanometers and a peak power preferably greater than onemilliwatt. The tunable source means may include optical amplifiers,which may take the form of semiconductor or optical fiber amplifiers.The tunable source means may also include means for defining a specificstate of polarization of the light emitted. The light emitted by thetunable source means may be any electromagnetic radiation at wavelengthsappropriate for the measurements being made and the optical fiber undertest. Preferably, the wavelength of the tunable source means is in therange of approximately 1100 nanometers to 1800 nanometers. Thewavelength of the tunable source means maybe varied by any methodappropriate for the embodiment of the tunable source means, such as byvarying the temperature of a semiconductor laser diode. Preferably, thewavelength of the tunable source means is varied by increments ofwavelength which may be a small fraction of the wavelength bandwidth,and over a range which may be substantially greater than the wavelengthbandwidth. More preferably, the wavelength of the tunable source isvaried by increments of less than 0.1 nanometers over a range of greaterthan 10 nanometers.

The optical connector means may comprise of one of the many opticalfiber connectors commonly used to join two optical fibers. Preferably,the optical connector means has very low polarization dependent loss.

The photodetector means may comprise one or more PIN diodes, avalanchephotodiodes, phototransistors, or photomultipliers.

The launch controller means, receive controller means, and processormeans may comprise personal computers, microprocessors, dedicatedelectronic processors, analogue to digital convertors, digital toanalogue convertors, electric motors, andlor other electronic andelectromechanical components, as are well-known to someone skilled inthe art of measurement instrumentation. The processor means may includeprocessing algorithms which may be related to the physical theory ofmeasurement, such as for polarization mode dispersion. The processormeans may also include algorithms which may be appropriate only for aspecific optical fiber and specific conditions, where the measurementproblem may not be fully tractable to known theoretical analysis, but,nevertheless, measurements obtained by the current invention may becorrelated with the optical fiber to provide useful information aboutthe behaviour of the specific optical fiber.

In a first embodiment of the present invention, the apparatus is one inwhich measurements of the state of polarization of the backscatteredlight are made consecutively, and such apparatus is one in which thereis only one separate channel of light output from the output port of thepolarization selecting coupler means, such that the photodetector meansconverts the intensity of the said one separate channel of light; and inwhich the polarization selecting coupler means comprises:

polarization independent coupler means which comprises an input port, abi-directional port and an output port, and which is for conveying lightbetween the input port, the bi-directional port and the output port,with negligible polarization dependent loss of the light, such thatlight input at the input port becomes light output from thebi-directional port, light input at the bi-directional port becomeslight output from the output port, and the light is substantiallyunchanged as it passes in either direction between the bi-directionalport of the polarization selecting coupler means and the bi-directionalport of the polarization independent coupler means;

polarization controller means for converting the state of polarizationof light input at the input port of the polarization selecting couplermeans into a particular launch state of polarization of light input atthe input port of the polarization independent coupler means, such thatthe particular launch state of polarization is selected as specified bythe launch controller means; and

polarization analyzer means for selecting a particular receive state ofpolarization of the light output from the output port of thepolarization independent coupler means to become the one separatechannel of light output from the output port of the polarizationselecting coupler means, such that the particular receive state ofpolarization is selected as specified by the receive controller means.

The polarization independent coupler means may include one or morebulk-optic components, such as non-polarising beam-splitters, or mayinclude one or more optical fiber components, such as directional fibercouplers or optical fiber circulators. In addition, the polarizationindependent coupler means may include an active element such as anacousto-optic deflector.

The polarization controller means may comprise one or more bulk-opticbirefringent waveplates, such as half-wave and quarter-wave plates, andlinear or circular polarizers, that may be rotated about an optical axisin an optical path, and/or inserted and removed from the optical path,to select a specific state of polarization of the light output from thepolarization controller means. The polarization controller means mayalso comprise contiguous sections of birefringent optical fiber that maybe positioned relative to each other to alter the state of polarizationof the light output. The polarization controller means may also includeone or more planar integrated optics circuits in which the state ofpolarization of the light output may be selected by electrical signalssupplied by the launch controller means.

The polarization analyzer means may comprise one or more bulk-opticbirefringent waveplates, such as half-wave and quarter-wave plates, andlinear or circular polarizers, that may be rotated about an optical axisin an optical path, and/or inserted and removed from the-optical path,to select a specific state of polarization of the light input from thepolarization independent coupler means. The polarization analyzer meansmay also include one or more planar integrated optics circuits in whichthe state of polarization of the light output is selected by electricalsignals supplied by the receiver controller means.

In the first embodiment of the present invention, the apparatus may beone in which the launch controller means specifies, and the polarizationcontroller means selects, only two particular launch states ofpolarization at each of the wavelengths of the tunable source meanscontrolled by the launch controller means.

In the first embodiment of the present invention, the apparatus mayalternatively be one in which the launch controller means specifies, andthe polarization controller means selects, only one separate particularlaunch state of polarization at each of the wavelengths of the tunablesource means controlled by the launch controller means. The polarizationcontroller means may comprise a single bulk-optic component, such as alinear polarizer or a circular polarizer.

In the first embodiment of the present invention, the apparatus may beone in which the receive controller means specifies, and thepolarization analyzer means selects, only two particular receive statesof polarization of the light output from the output port of thepolarization independent coupler means for each of the particular launchstates of polarization of the light input at the input port of thepolarization independent coupler means at each of the wavelengths of thetunable source means; and is one in which the processor means is suchthat the state of polarization of the light backscattered by the opticalfiber is deduced from the two separate particular receive states ofpolarization.

In a second embodiment of the present invention, the apparatus is one inwhich the polarization controller means comprises an input polarizermeans for selecting a single particular launch state of polarization forthe light input to the input port of the polarization independentcoupler means, and the polarization analyzer means comprises an outputpolarizer means for selecting a single particular receive state ofpolarization of the light output from the output port of thepolarization independent coupler means. The input polarizer means maycomprise a linear polarizer, and the output polarizer means may comprisea linear polarizer.

In a third embodiment of the present invention, the apparatus is one inwhich measurements of the state of polarization of the backscatteredlight are made consecutively, and such apparatus is one in which thereis only one separate channel of light output from the output port of thepolarization selecting coupler means, such that the photodetector meansconverts the intensity of the one separate channel of light; and inwhich the polarization selecting coupler means comprises:

polarization independent coupler means which comprises an input port, abi-directional port and an output port, and which is for conveying lightbetween the input port, the bi-directional port and the output port,with negligible polarization dependent loss of the light, such thatlight input at the input port becomes light output from thebi-directional port, light input at the bi-directional port becomeslight output from the output port, the light is substantially unchangedas it passes from the input port of the polarization selecting couplermeans to the input port of the polarization independent coupler means,and the light is substantially unchanged as it passes from the outputport of the polarization independent coupler means to the output port ofthe polarization selecting coupler means; and

bi-directional polarizer means for selecting a particular state ofpolarization of the light that passes in either direction between thebi-directional port of the polarization independent coupler means andthe bi-directional port of the polarization selecting coupler means.

In the third embodiment of the present invention, the bi-directionalpolarizer means may comprise a linear polarizer.

In a fourth embodiment of the present invention, the apparatus is one inwhich measurements of the state of polarization of the backscatteredlight may be made in parallel, and such apparatus is one in which thereare two or more separate channels of light output from the output portof the polarization selecting coupler means, such that the photodetectormeans converts the intensities of the said two or more separate channelsof light, and such that the polarization selecting coupler meanscomprises:

polarization independent coupler means which comprises an input port, abi-directional port and an output port, and which is for conveying lightbetween the input port, the bi-directional port and the output port,with negligible polarization dependent loss of the light, such thatlight input at the input port becomes light output from thebi-directional port, light input at the bi-directional port becomeslight output from the output port, and the light is substantiallyunchanged as it passes in either direction between the bi-directionalport of the polarization selecting coupler means and the bi-directionalport of the polarization independent coupler means;

polarization controller means for converting the state of polarizationof light input at the input port of the polarization selecting couplermeans into a particular launch state of polarization of light input atthe input port of the polarization independent coupler means, such thatthe particular launch state of polarization is selected as specified bythe launch controller means; and

polarization parallel analyzer means for selecting two or moreparticular receive states of polarization of the light output from theoutput port of the polarization independent coupler means to become thetwo or more separate channels of light output from the output port ofthe polarization selecting coupler means.

In the fourth embodiment of the present invention, the polarizationcontroller means may be such that only two separate particular launchstates of polarization may be selected at each of the wavelengths of thetunable source means controlled by the launch controller means, or,alternatively, the polarization controller means may be such that onlyone separate particular launch state of polarization may be selected ateach of the wavelengths of the tunable source means controlled by thelaunch controller means.

The polarization parallel analyzer means may include bulk-opticbirefringent waveplates, such as half-wave and quarter-wave plates, andlinear or circular polarizers, that may be placed in two or moreseparate optical paths, to select two or more particular receive statesof polarization of the light input from the polarization independentcoupler means, such that the light output comprises two or more separatechannels. The polarization parallel analyzer means may also include oneor more planar integrated optics circuits in which two or more receivestates of polarization of the light output in two or more separatechannels may be selected by electrical signals supplied by the receivercontroller means.

In the fourth embodiment of the present invention, the apparatus may beone in which the polarization parallel analyzer means comprises a beamseparator means for separating the light input into two or more separatebeams with negligible polarization dependent loss, and comprises two ormore single polarizer means for selecting a particular receive state ofpolarization for each of the said two or more separate beams.

The two or more single polarizer means may be such that a first singlepolarizer means comprises a linear polarizer, such that a second singlepolarizer means also comprises a linear polarizer but with a differentorientation of its polarization axis, and such that a third singlepolarizer means comprises a circular polarizer.

Alternatively, in the fourth embodiment of the present invention, theapparatus may be one in which the polarization parallel analyzer meansselects only two particular receive states of polarization of the lightoutput from the output port of the polarization independent couplermeans for each of the separate particular launch states of polarizationlight input at the input port of the polarization independent couplermeans at each of the wavelengths of the tunable source means; and is onein which the processor means is such that the state of polarization ofthe light backscattered by the optical fiber is deduced from the twoparticular receive states of polarization.

In this alternative embodiment of the present invention, the apparatusmay be one in which the polarization parallel analyzer means comprises abeam separator means for separating the light input into only twoseparate beams, and in which the apparatus includes only two singlepolarizer means for selecting a particular receive state of polarizationfor each of the said two separate beams.

The two single polarizer means may be such that a first single polarizermeans comprises a linear polarizer, and such that a second singlepolarizer means also comprises a linear polarizer but with a differentorientation of its polarization axis, or such that the second singlepolarizer means comprises a circular polarizer.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a block diagram which shows first apparatus for measuringcharacteristics of an optical fiber, and illustrates the relationshipsbetween the tunable source means, the polarization selecting couplermeans, the optical connector means, the optical fiber, the photodetectormeans, the launch controller means, the receive controller means, andthe processor means;

FIG. 2 is a block diagram which shows apparatus of the present inventionin which the polarization controller means, the polarization independentcoupler means, and the polarization analyzer means are shown for onlyone separate channel of light output from the output port of thepolarization selecting coupler means;

FIG. 3 is a block diagram which shows apparatus of the present inventionin which the polarization selecting coupler means includes the inputpolarizer means and the output polarizer means;

FIG. 4 is a block diagram which shows apparatus of the present inventionin which the polarization selecting coupler means includes thebi-directional polarizer means;

FIG. 5 is a block diagram of an apparatus of the present invention inwhich the polarization controller means, the polarization independentcoupler means, and the polarization parallel analyzer means are shownfor two or more separate channels of light output from the output portof the polarization selecting coupler means; and

FIG. 6 is a block diagram of an apparatus of the present invention inwhich the polarization parallel analyzer means includes two or moresingle polarizer means.

SPECIFIC DESCRIPTION

FIG. 1 shows a launch controller means 1 which selects the wavelength ofa tunable source means 2 and the timing parameters of a sequence oflaunch optical pulses 3 via a communications link 20. The launch opticalpulses 3 enter a polarization selecting coupler means 5 at an input port4 and are output at a bi-directional port 6 as launch optical pulses 7with a particular launch state of polarization specified by the launchcontroller means 1 and selected by the polarization selecting couplermeans 5 via the communications link 20. The launch optical pulses 7 passthrough an optical connector means 8 and enter the near end 10 of anoptical fiber 11 as launch optical pulses 9. The optical connector means8 facilitates the optical connection between the optical fiber 11 andthe polarization selecting coupler means 5. As the launch optical pulses9 pass along the optical fiber 11, some light is backscattered by thephysical process of Rayleigh scattering, and returns back along theoptical fiber 11, arriving at the near end 10 as backscattered light 13with a specific state of polarization, determined by the state ofpolarization of the launch optical pulses 9 and the polarizationcharacteristics of the route taken through the optical fiber 11. Thebackscattered light 13 passes back through optical connector means 8 toenter the bi-directional port 6 of the polarization selecting couplermeans 5 as backscattered light 14. One or more particular receive statesof polarization of the backscattered light 14 are selected by thepolarization selecting coupler means 5 via the communications link 20 asthe backscattered light 14 passes from bi-directional port 6 to outputport 15 to become one or more separate channels of light 16. A receivecontroller means 18 specifies, via the communications link 20, and thepolarization selecting coupler means 5 selects a particular receivestate of polarization of the backscattered light 14 to be output in eachof the separate channels of light 16. The photodetector means 17converts the intensity of the light in each of the separate channels oflight 16 into separate electrical signals which are controlled by thereceive controller means 18 so that they are transferred to a processormeans 19 by the communications link 20 at selected intervals of timerelative to the launch optical pulses 3. These intervals of timedetermine the position in length within the optical fiber 11 from whichthe light 16 was backscattered. In this way, the processor means 19collects a set of data consisting of the state of polarization of thebackscattered light 13 as a function of the state of polarization of thelaunch optical pulses 9, a function of the position in length within theoptical fiber 11, and a finction of the wavelength of the launch opticalpulses 3 generated by the tunable source means 2. The processor means 19may then compute a range of different characteristics of the opticalfiber 11, including beat length, correlation length, and polarizationmode dispersion. Polarization mode dispersion may be computed by anumber of well-known methods, some of which are detailed below withreference to other embodiments of the present invention.

Referring to FIG. 2, the polarization selecting coupler means 5 is suchthat only one channel of light 41 is output from the output port 15. Thelaunch optical pulses 3 enter input port 4 to become launch opticalpulses 30 which enter polarization controller means 31. The launchcontroller means 1 specifies, and the polarization controller means 31selects, a particular launch state of polarization for the launchoptical pulses 32 which leave the polarization controller means 31 andenter a polarization independent coupler means 34 through input port 33.The launch optical pulses 32 are output from the polarizationindependent coupler means 34 at a bi-directional port 35 as launchoptical pulses 36 and then output from bi-directional port 6 as launchoptical pulses 7, having incurred negligible polarization dependentloss. The backscattered light 14 enters through the bi-directional port6 to become backscattered light 21 which enters the bi-directional port35 and is output from the output port 37 as backscattered light 38,having incurred negligible polarization dependent loss. A polarizationanalyzer means 39 selects a particular receive state of polarization ofbackscattered light 38 to be output as backscattered light 40, where theparticular receive state of polarization is specified by the receivecontroller means 18 via the communications link 20. The backscatteredlight 40 is output from the output port 15 as backscattered light 41,and the intensity of backscattered light 41 is then measured byphotodetector means 17. In a typical measurement sequence determined bythe processor means 19, one or more particular receive states ofpolarization of backscattered light 38 may be selected, and theintensity of backscattered light 41 measured, consecutively, at each ofone or more particular launch states of polarization selected by thepolarization controller means 31 at each of the selected wavelengths oftunable source means 2, over a range of timing intervals which representa range of length position within the optical fiber 11. A complete stateof polarization of the backscattered light 13 may be computed by theprocessor means 19 from the measurements made by the photodetector means17 for three or more separate particular receive states of polarizationselected by the polarization analyzer means 39. Polarization modedispersion may be computed by the well-known method of Jones MatrixEigenanalysis. As can be appreciated from FIG. 2, at each of twoseparate wavelengths of tunable source means 2, a Jones matrix of theoptical fiber 11 is obtained by measuring three complete states ofpolarization of the backscattered light 13 for three separate particularlaunch states of polarization of the launch optical pulses 9 and using aprocessing algorithm such as that described in, for example, R. C.Jones, “A new calculus for the treatment of optical systems VI:Experimental determination of the Matrix”, J. Optical.Soc.Amer., Vol.37,No.2, pp110-112, 1947. The two Jones matrices at the two separatewavelengths are then processed by an algorithm such as that describedin, for example, ANSI/TIA/EIA-455-122-1996, “FOTP-122 Polarization ModeDispersion Measurement for Single-Mode Optical Fibers by Jones MatrixEigenanalysis”, to give the polarization mode dispersion as a singlevalue for the wavelength range defined by the two wavelengths at whichthe measurements were made. It may be assumed that the range ofwavelengths in the backscattered light 13 may be small enough for thebackscattered light 13 to be considered to be fully polarized.

In another embodiment of the present invention, and also with referenceto FIG. 2, the launch controller means 1 specifies and the polarizationcontroller means 31 selects only two separate particular launch statesof polarization for the launch optical pulses 9 so that only twoseparate complete states of polarization of the backscattered light 13are measured at each of the wavelengths of the tunable source means 2selected by the launch controller means 1. Although two complete statesof polarization are insufficient to define a Jones matrix of the opticalfiber 11, it is possible to compute the polarization mode dispersionusing the well-known Poincaré sphere method as described in, forexample, N. S. Bergano, C. D. Poole, R. E. Wagner, “Investigation ofPolarization Dispersion in Long Lengths of Single-Mode Fiber usingMultilongitudinal Mode Lasers”, IEEE Journal of Lightwave Technology,Vol.LT-5, No.11, pp. 1618-1622, 1987. It is well-known that a completestate of polarization of light may be described by a Stokes vector,which may be visualized as a line connecting a point on the surface of asphere of unit radius to the center of the sphere where the sphere iscalled the Poincaré sphere, as described in, for example, D. S. Kliger,J. W. Lewis, C. E. Randall, “Polarised Light in Optics andSpectroscopy”, Academic Press, ISBN 0-12-414975-8, 1990. The radius ofunity for the Poincaré sphere follows from the assumption that the rangeof wavelengths in the backscattered light 13 is small enough for thebackscattered light 13 to be considered to be fully polarized. At anyone wavelength of the tunable source means 2, and for timing ofmeasurement by the receive controller means 18 so that the backscatteredlight 13 comes from a specific length position in the optical fiber 11,the Stokes vector of the measured state of polarization of backscatteredlight 13 describes a great circle plane of the Poincaré sphere, as thestate of polarization of the launch optical pulses 9 is changed by thepolarization controller means 31. A great circle is defined as being acircle on the surface of a sphere whose center coincides with the centerof the circle. Accordingly, the great circle plane may be defineduniquely by the measurement of just two complete states of polarizationof backscattered light 13 for two separate, preferably orthogonal,states of polarization of launch optical pulses 9. At two differentwavelengths of the tunable source means 2, two different great circleplanes may be obtained, and these plane intersect in a diameter of thePoincaré sphere. The two points where this diameter intersects with thesurface of the sphere are called the principal states of polarization,and may be related to a well-known mathematical formulation as describedin, for example, C. D. Poole, R. E. Wagner, “Phenomenological approachto polarization dispersion in long single-mode fibers”, ElectronicsLetters, Vol.22, No.19, pp1029-1030, 1986. Accordingly, the great circleplane can be visualized as rotating about the principal states ofpolarization as the wavelength changes, and it is well-known that theangle of rotation leads directly to the polarization mode dispersion, asdescribed in, for example, C. D. Poole, N. S. Bergano, R. E. Wagner, H.J. Schulte, “Polarization Dispersion and Principal States in a 147-kmUndersea Lightwave Cable”, IEEE Journal of Lightwave Technology, Vol.6,No.7, pp1185-1190, 1988.

In another embodiment of the present invention, and also with referenceto FIG. 2, the launch controller means 1 specifies and the polarizationcontroller means 31 selects only one separate particular launch state ofpolarization for the launch optical pulses 9 so that only one completestate of polarization of the backscattered light 13 is measured at eachof the wavelengths of the tunable source means 2 selected by the launchcontroller means 1. In the description of the previous embodiment of thepresent invention, it was described how the polarization mode dispersionwas obtained from the angle between two great circle planes, eachmeasured at a separate wavelength. A single Stokes vector in one ofthese planes will describe a small circle on the surface of the Poincarésphere as the wavelength changes and the great circle plane rotates,where the center of the small circle lies on the diameter that is theintersection of the great circle planes which links the principal statesof polarization. Three separate measurements of position on the smallcircle, obtained from three separate measurements of the complete stateof polarization of backscattered light 13 at three separate wavelengthsof tunable source means 2, are sufficient to define the center of thesmall circle and thus the axis of rotation of the great circle planes,as shown in, for example, D. Andresciani, F. Curti, F. Matera, B. Daino,“Measurement of group-delay difference between the principal states ofpolarization on a low-birefringence terrestrial fiber cable”, OpticsLetters, Vol.12, pp.844-846, 1987. The polarization mode dispersion maythen be computed from the angle of rotation of the great circle planes.

In another embodiment of the present invention, and also with referenceto FIG. 2, the polarization analyzer means 39 selects only two separateparticular receive states of polarization of the backscattered light 13which are measured at each of the wavelengths of the tunable sourcemeans 2 selected by the launch controller means 1. It is well-known thatthe complete state of polarization of light is described by threeindependent parameters, representing the amplitude of the electricalvector in two orthogonal polarizations and the phase angle between thetwo, which leads to three independent components for a Stokes vector forfully polarized light. Thus, three or more separate measurements arerequired to obtain the three independent components. However, withreference to the Poincaré sphere, the modulus of the magnitude of onecomponent may be deduced from independent measurements of the other twocomponents by the constraint that the magnitude of the Stokes vector isunity and hence the sum of the squares of the magnitudes of all threecomponents must equal unity, where the factor of unity follows from theassumption that the range of wavelengths in the backscattered light 13is small enough for the backscattered light 13 to be considered to befully polarized. With respect to visualization on the Poincaré sphere,this means that a Stokes vector is known to the uncertainty that thethird component may be at either end of a diameter of the sphere. Ingeneral, this is not sufficient to fully characterize the complete stateof polarization. However, for some optical fibers 11, the rate ofrotation of the great circle planes about the intersecting diameter issmall enough, as the wavelength of tunable source means 2 charges, suchthat the relative change in the Stokes vector is small enough to removeany ambiguity caused by the uncertainty in sign of the deduced componentof the state of polarization of the backscattered light 13. Accordingly,it becomes possible to estimate the polarization mode dispersion by theJones Matrix Eigenanalysis method, the Poincaré sphere great circlemethod, or the Poincaré sphere small circle method as previouslydescribed. For the embodiments of the present invention previouslydescribed, the measurement of the polarization mode dispersion dependsupon two or more measurements made at two or more separate wavelengthsof the tunable source means 2. Preferably, the range of wavelengths inthe launch optical pulses 3 may be less than the wavelength intervalbetween the measurements, so that accurate results may be obtained fromthe wavelength independence of the measurements. In a preferredembodiment of the present invention, the wavelength bandwidth of thetunable source means 2 may be less than 0.1 nanometers and the smallestspacing of wavelength selectable by launch controller means 1 may beless than 0.1 nanometers, which may be particularly advantageous forinvestigating the change in polarization mode dispersion over the rangeof a few tens of nanometers of wavelength that are of current interestfor wavelength division multiplexed optical communication systems.

For the embodiments of the present invention previously described, threedifferent methods may be used to obtain a measurement of the sameparameter, polarization mode dispersion. Each method is well-known andused for the measurement of the polarization mode dispersion where thestate of polarization of the light passing from one end of an opticalfiber to the other is measured, and so the light has made a single passthrough the fiber. However, a particular characteristic of the presentinvention is that the light measured has traversed the optical fiber 11in one direction, been backscattered at a particular point in theoptical fiber 11, and then returned back through the same length ofoptical fiber 11. Accordingly, the measurement of the polarization modedispersion made by the present invention, and described above, is thatof the combined forward and backward pass of the light through theoptical fiber 11. In general, the most useful parameter may be that ofthe polarization mode dispersion between one end of a fiber and theother end, and so it may be necessary to understand the relationshipbetween the two measurements. The use of polarized light to study thecharacteristics of light backscattered from an optical fiber has beenwell-known since early work in the field of polarization optical timedomain reflectometry (POTDR) was published, such as, for example, A. J.Rogers, “Polarization optical time domain reflectometry”, ElectronicsLetters, Vol.16, No.13, pp.489-490, 1980. It is also well-known thatcircular birefringence in the fiber is not detectable in a simple anddirect way by POTDR as shown in, for example, J. N. Ross, “Birefringencemeasurement in optical fibers by polarization-optical time-domainreflectometry”, Applied Optics, Vol.21. No.19, pp.3489-3495, 1982. Theinventor of the present invention has studied this problem and hasconcluded that the polarization mode dispersion of an optical fiber fromone end to the other end may be equal to the polarization modedispersion, when measured by light which has made a forward and abackward pass through the optical fiber, multiplied by a factor 0.64, onaverage. This numerical value has been obtained from many numericalsimulations using mathematical models of optical fibers, as describedin, for example, F. Curti, B. Daino, Q. Mao, F. Matera, C. G. Someda,“Concatenation of Polarization Dispersion in Single-Mode Fibres”,Electronics Letters, Vol.25, No.4, pp.290-292, 1989. It is believed thatthis value is different from the value of 0.707, which would be expectedfor the polarization mode dispersion of two concatenated butuncorrelated lengths of fiber, because the light may be correlatedbetween the forward and backward passes as each section of the opticalfiber is always traversed twice, once in each direction. The uncertaintyin the numerical factor of 0.64 is expected to be much less than thevariation in the measured polarization mode dispersion arising from ageneral statistical Maxwellian distribution, which is typical of modemsingle-mode optical fibers, as discussed in, for example, F. Curti, B.Daino, G. Marchis, F. Matera, “Statistical Treatment of the Evolution ofthe Principal States of Polarization in Single-Mode Fibers”, IEEEJournal of Lightwave Technology, Vol.8, No.8, pp 1162-1166, 1990.

The embodiments of the present invention, as previously described,obtain one or more states of polarization as a function of bothwavelength of the tunable source 2 and length position within theoptical fiber 11. Thus, at each wavelength the complete state ofpolarization as a finction of length position within the optical fiber11 may be derived. This function may be interpreted as resulting in anestimate of the beat length within the optical fiber, where thebirefringence of the fiber is approximately constant, as described in,for example, A. Galtarossa, G. Gianello, C. G. Someda, M. Schiano,“Stress Investigation in Optical Fiber Ribbon Cable by Means ofPolarization Sensitive Techniques”, IEEE Photonics Technology Letters;Vol.6, No.10, pp.1232-1235, 1994. Alternatively, where the states ofpolarization are changing rapidly with length position arising fromshort range fluctuations in birefringence in the optical fiber, it maybe appropriate to autocorrelate the states of polarization to arrive ata correlation length which estimates how rapidly the fluctuations in thebirefringence takes place. A particular advantage of the presentinvention is that the complete state of polarization of thebackscattered light is measured, and this is independent of the absoluteintensity of the backscattered light, which means that the loss ofoptical power which occurs as the light traverses forward and then backthrough the optical fiber 11 is not important to the measurement ofcharacteristics such as polarization mode dispersion. However, theabsolute intensity of the backscattered light may be known, just as witha conventional optical time domain reflectometer, and so it may bepossible to measure the optical power loss of the optical fiber 11 atthe same time as making the measurements of the states of polarization.

Referring now to FIGS. 2 and 3, the polarization controller means 31comprises an input polarizer means 42, which selects a single particularlaunch state of polarization for the light 32 input to the input port 33of the polarization independent coupler means 34. The input polarizermeans 42 may be a linear polarizer. In addition, the polarizationanalyzer means 39 comprises an output polarizer means 43 which selects asingle particular receive state of polarization of the light output 38from the output port 37 of the polarization independent coupler means34. The output polarizer means 43 may be a linear polarizer. Thisembodiment of the present invention provides only one particular launchstate of polarization of the launch optical pulses 9 and measures theintensity of only one particular receive state of polarization of thebackscattered light 13, at each of the wavelengths of the tunable sourcemeans 2 and the length position in the optical fiber 11, determined bylaunch controller means 1 and receive controller means 20. Thismeasurement of only one particular receive state of polarization of thebackscattered light 13 is insufficient for the determination of thegreat circle planes within the Poincaré sphere representation asdescribed previously. However, it is well-known that over a sufficientrange of wavelengths the states of polarization of the backscatteredlight 13 will give Stokes vectors that cover most of the surface of thePoincaré sphere, and it then becomes possible to use the well knownfixed-polarizer wavelength-scanning method of computing the approximatevalue of the polarization mode dispersion, as described in, for example,C. D. Poole, D. L. Favin, “Polarization-Mode Dispersion MeasurementsBased on Transmission Spectra Through a Polarizer”, IEEE Journal ofLightwave Technology, Vol.12, No.6, pp917-929, 1994.

Referring to FIGS. 1 and 4, the launch optical pulses 3 are input to theinput port 4 of the polarization selecting coupler means 5 to becomelaunch optical pulses 23. The launch optical pulses 23 are input to theinput port 33 of the polarization independent coupler means 34 and areoutput from the bi-directional port 35 as launch optical pulses 36,having incurred negligible polarization dependent loss. Bi-directionalpolarizer means 46 selects one particular launch state of polarizationof launch optical pulses 36 to become launch optical pulses 45 which areoutput from the bi-directional port 6 as the launch optical pulses 7.The backscattered light 14 is input to bi-directional port 6 to becomethe backscattered light 21 which returns through the bi-directionalpolarizer means 46 to become backscattered light 47 with one particularreceive state of polarization. The backscattered light 47 is input tothe bi-directional port 35 of the polarization independent coupler means34 and is output from the output port 37 as backscattered light 24 whichbecomes the backscattered light 41 output from output port 15, havingincurred negligible polarization dependent loss. In this way, thebi-directional polarizer means 46 selects both the particular launchstate of polarization of the launch optical pulses 7, and the particularreceive state of polarization of the backscattered light 41. Thebi-directional polarizer means 46 may be a linear polarizer which may beof particular value for a portable and low cost embodiment of thepresent invention.

As with measuring polarization mode dispersion using the Jones MatrixEigenanalysis method and the Poincaré sphere methods, describedpreviously, the value of polarization mode dispersion measured using thefixed polarizer wavelength scanning method, described previously, needsto be corrected to take into account the correlation between the lightas it passes forward through the optical fiber 11 and as it returnsafter being backscattered. The inventor of the present invention hasstudied this problem and has concluded that the polarization modedispersion of the fiber from one end to the other end may be equal tothe polarization mode dispersion, when measured by light which has madea forward and a backward pass through the optical fiber, multiplied bythe factor 0.60, on average. This numerical value has been obtained frommany numerical simulations using mathematical models of optical fibers,as described in, for example, F. Curti, B. Daino, Q. Mao, F. Matera, C.G. Someda, cited previously, and over the same range of models as usedto obtain the numerical factor of 0.64 cited previously. The uncertaintyin the numerical factor of 0.60 is expected to be much less than thevariation in the measured polarization mode dispersion because of thenature of the statistical Maxwellian distribution as described in, forexample, F. Curti, B. Daino, G. Marchis, F. Matera, cited previously.

Referring to FIG. 5, the polarization selecting coupler means 5 is suchthat two or more separate channels of light 25 are output from theoutput port 15. The launch optical pulses 3 enter input port 4 and areoutput from bi-directional port 6 to become the launch optical pulses 7with a particular launch state of polarization selected by thepolarization controller means 31, and the backscattered light 13 becomesthe backscattered light 38, as previously described. The backscatteredlight 38 is input to an input port 48 of a polarization parallelanalyzer means 49 and is separated into two or more separate channels oflight 22 and output from an output port 50, and which are output fromthe output port 15 as the separate channels of light 25. Each of theseparate channels of light 22 results from the selection of a separateparticular receive state of polarization of the backscattered light 38,and so photodetector means 17 may convert the intensities of thechannels of light 25 into separate electrical signals in parallel. Thecapability of this embodiment of the present invention of makingmeasurements in parallel may be applied to all the different methods ofmeasuring the complete state of polarization of the backscattered light13 described previously, and may be particularly advantageous becausethere may be no need to make measurements of the individual componentsof the complete state of polarization of the backscattered light 13consecutively, and so the time taken to make an overall measurement maybe reduced.

Referring to FIGS. 5 and 6, the polarization parallel analyzer means 49includes a beam separator means 52. Light 51 input at the input port 48is separated by the beam separator means 52 into two or more separatebeams 53 with negligible polarization dependent loss. Each of theseparate beams 53 passes through a separate single polarizer means 54 tobecome a separate output beam 55, each with a particular receive stateof polarization, and where the output beams 55 are output from an outputport 50 as the separate channels of light 22.

In a further embodiment of the present invention, there are three of thesingle polarizer means 54, in which a first one of the single polarizermeans 54 comprises a linear polarizer, in which a second one of thesingle polarizer means 54 also comprises a linear polarizer but with adifferent orientation of its polarization axis, preferably orthogonal,and in which a third one of the single polarizer means 54 compnses acircular polarizer. The resulting three output beams 55 may be measuredin parallel to provide a complete state of polarization of thebackscattered light 13.

In a further embodiment of the present invention, there are two of thesingle polarizer means 54, in which a first one of the single polarizermeans 54 comprises a linear polarizer, and in which a second one of thesingle polarizer means 54 also comprises a linear polarizer but with adifferent orientation of its polarization axis, preferably orthogonal.Alternatively, the first one of the single polarizer means 54 comprisesa linear polarizer, and the second one of the single polarizer means 54comprises a circular polarizer. The resulting two output beams 55 may bemeasured in parallel to provide an estimate of a complete state ofpolarization of the backscattered light 13.

In any embodiment of the present invention the optical components may besuch that there may be a residual change in state of polarization whichis added to that resulting from the optical fiber 11. The residualchange in state of polarization may be measured by a process ofcalibration in which measurements may be made with the present inventionfor a particular known optical fiber 11, or with an artefact substitutedfor optical fiber 11, whose optical transfer function is known. Thecalibration measurement may then be used to correct a subsequentmeasurement of an unknown optical fiber 11, and the calibration processmay be carried out as often as required to obtain a satisfactorymeasurement of the residual change.

It is to be appreciated that the embodiments of the invention describedabove with reference to the accompanying drawings have been given by wayof example only. Thus, parts of the illustrated embodiments may becombined in any suitable appropriate order. Also, modifications andadditional components may be provided to enhance the performance of theapparatus. Thus, for example, the apparatus may include means forreducing chirping, that is, the variation in wavelength of the tunablesource means 2 throughout the duration of each of the launch opticalpulses 3. The apparatus may include means to guide and form the light asit passes through the apparatus. Such means may include opticalcomponents, such as lenses and mirrors. The apparatus may also includemeans to improve the signal to noise ratio of the measurements. Suchmeans may include optical components to reduce stray light, or to reduceundesirable changes in the polarization properties of the apparatus ofthe present invention.

I claim:
 1. An apparatus for measuring characteristics of an opticalfiber, at different positions along the length of the optical fiber,which apparatus comprises: tunable source means for providing opticalpulses of light which have a variable wavelength and a narrow wavelengthbandwidth; polarization selecting coupler means which comprises an inputport, a bidirectional port and an output port, and which is forconveying light between the input port, the bi-directional port and theoutput port, such that a state of polarization of the optical pulses oflight input at the input port becomes a particular launch state ofpolarization of light output from the bidirectional port, and such thatone or more particular receive states of polarization of light input atthe bi-directional port become one or more separate channels of lightoutput from the output port; optical connector means for making anoptical connection between the bidirectional port of the polarizationselecting coupler means and one end of the optical fiber, so that thelight output from the bi-directional port of the polarization selectingcoupler means is launched into the optical fiber and the lightbackscattered within the optical fiber is received as the light input atthe bi-directional port of the polarization selecting coupler means;photodetector means for converting the intensity of each of the one ormore separate channels of light output form the output port of thepolarization selecting coupler means into one or more separateelectrical signals; launch controller means for controlling the timing,duration and wavelength of the optical pulses provided by the tunablesource means, and for specifying the particular launch state ofpolarization of the light output from the bi-directional port of thepolarization selecting coupler means; receive controller means forcontrolling the timing of measuring the electrical signals provided bythe photodetector means, and for specifying the one or more particularreceive states of polarization of light input at the bi-directional portof the polarization selecting coupler means; and processor means formeasuring and processing the electrical signals provided by thephotodetector means into measurements of the characteristics of theoptical fiber, and wherein there is, only one separate channel of lightoutput from the output port of the polarization selecting coupler means,such that the photodetector means converts the intensity of the said oneseparate channel of light; and in which the polarization selectingcoupler means comprises: polarization independent coupler means whichcomprises an input port, a bi-directional port and an output port, andwhich is for conveying light between the input port, the bi-directionalport and the output port, with negligible polarization dependent loss ofthe light, such that light input at the input port become light outputfrom the bi-directional port, light input at the bi-directional portbecomes light output from the output port, and the light issubstantially unchanged as it passes in either direction between thebidirectional port of the (polarization) polarization selecting couplermeans and the bi-directional port of the polarization independentcoupler means; polarization controller means for converting the state ofpolarization of light input at the input port of the polarizationselecting coupler means into a particular launch state of polarizationof light input at the input port of the polarization independent couplermeans, such that the particular launch state of polarization is selectedas specified by the launch controller means; and polarization analyzermeans for selecting a particular receive state of polarization of thelight output from the output port of the polarization independentcoupler means to become the one separate channel of light output fromthe output port of the polarization selecting coupler means, such thatthe particular receive state of polarization is selected as specified bythe receive controller means.
 2. The apparatus according to claim 1 inwhich the launch controller means specifies, and the polarizationcontroller means selects, only two particular launch states ofpolarization at each of the wavelengths of the tunable source meanscontrolled by the launch controller means.
 3. The apparatus according toclaim 1 in which the launch controller means specifies, and thepolarization controller means selects, only one particular launch stateof polarization at each of the wavelengths of the tunable source meanscontrolled by the launch controller means.
 4. The apparatus according toclaim 1 in which the receive controller means specifies, and thepolarization analyzer means selects, only two particular receive statesof polarization of the light output from the output port of thepolarization independent coupler means for each of the particular launchstates of polarization of the light input at the input port of thepolarization independent coupler means at each of the wavelengths of thetunable source means; and in which the processor means is such that thestate of polarization of the light backscattered by the optical fiber isdeduced from the two particular receive states of polarization.
 5. Theapparatus according to claim 1 in which the polarization controllermeans comprises an input polarizer means for selecting a singleparticular launch state of polarization for the light input to the inputport of the polarization analyzer means comprises an output polarizermeans for selecting a single particular receive state of polarization ofthe light output from the output port of the polarization independentcoupler means.
 6. The apparatus according to claim 5 in which the inputpolarizer means comprises a linear polarizer, and in which the outputpolarizer means comprises a linear polarizer.
 7. An apparatus formeasuring characteristics of an optical fiber, at different positionsalong the length of the optical fiber, which apparatus comprises:tunable source means for providing optical pulses of light which have avariable wavelength and a narrow wavelength bandwidth; polarizationselecting coupler means which comprises an input port, a bidirectionalport and an output port, and which is for conveying light between theinput port, the bi-directional port and the output port, such that astate of polarization of the optical pulses of light input at the inputport becomes a particular launch state of polarization of light outputfrom the bi-directional port, and such that one or more particularreceive states of polarization of light input at the bi-directional portbecome one or more separate channels of light output from the outputport; optical connector means for making an optical connection betweenthe bidirectional port of the polarization selecting coupler means andone end of the optical fiber, so that the light output from thebi-directional port of the polarization selecting coupler means islaunched into the optical fiber and the light backscattered within theoptical fiber is received as the light input at the bi-directional portof the polarization selecting coupler means; photodetector means forconverting the intensity of each of the one or more separate channels oflight output from the output port of the polarization selecting couplermeans into one or more separate electrical signals; launch controllermeans for controlling the timing, duration and wavelength of the opticalpulses provided by the tunable source means, and for specifying theparticular launch state of polarization of the light output from thebi-directional port of the polarization selecting coupler means; receivecontroller means for controlling the timing of measuring the electricalsignals provided by the photodetector means, and for specifying the oneor more particular receive states of polarization of light input at thebi-directional port of the polarization selecting coupler means; andprocessor means for measuring and processing the electrical signalsprovided by the photodetector means into measurements of thecharacteristics of the optical fiber, and wherein there is, only oneseparate channel of light output from the output port of thepolarization selecting coupler means, such that the photodetector meansconverts the intensity of the one separate channel of light; and inwhich the polarization selecting the coupler means comprises:polarization independent coupler means which comprises an input port, abi-directional port and an output port, and which is for conveying lightbetween the input port, the bi-directional port and the output port,with negligible polarization dependent loss of the light, such thatlight input at the input port becomes light output from thebidirectional port, light input at the bi-directional port becomes lightoutput from the output port, the light is substantially unchanged as itpasses from the input port of the polarization selecting coupler meansto the input port of the polarization independent coupler means, and thelight is substantially unchanged as it passes from the output port ofthe polarization independent coupler means to the output port of thepolarization selecting coupler means; and bi-directional polarizer meansfor selecting a particular state of polarization of the light thatpasses in either direction between the bidirectional port of thepolarization independent coupler means and the bi-directional port, ofthe polarization selecting coupler means.
 8. The apparatus according toclaim 7 in which the bi-directional polarizer means comprises a linearpolarizer.
 9. An apparatus for measuring characteristics of an opticalfiber, at different positions along the length of the optical fiber,which apparatus comprises: tunable source means for providing opticalpulses of light which have a variable wavelength and a narrow wavelengthbandwidth; polarization selecting coupler means which comprises an inputport, a bidirectional port and an output port, and which is forconveying light between the input port, the bi-directional port and theoutput port, such that a state of polarization of the optical pulses oflight input at the input port becomes a particular launch state ofpolarization of light output from the bi-directional port, and such thatone or more particular receive states of polarization of light input atthe bi-directional port become one or more separate channels of lightoutput from the output port; optical connector means for making anoptical connection between the bidirectional port of the polarizationselecting coupler means and one end of the optical fiber, so that thelight output from the bi-directional port of the polarization selectingcoupler means is launched into the optical fiber and the lightbackscattered within the optical fiber is received as the light input atthe bi-directional port of the polarization selecting coupler means;photodetector means for converting the intensity of each of the one ormore separate channels of light output from the output port of thepolarization selecting coupler means into one or more separateelectrical signals; launch controller means for controlling the timing,duration and wavelength of the optical pulses provided by the tunablesource means, and for specifying the particular launch state ofpolarization of the light output from the bi-directional port of thepolarization selecting coupler means; receive controller means forcontrolling the timing of measuring the electrical signals provided bythe photodetector means, and for specifying the one or more particularreceive states of polarization of light input at the bi-directional portof the polarization selecting coupler means; and processor means formeasuring and processing the electrical signals provided by thephotodetector means into measurements of the characteristics of theoptical fiber, and wherein there are two or more separate channels oflight output from the output port of the polarization selecting couplermeans, such that the photodetector means converts the intensities of thesaid two or more separate channels of light, and such that thepolarization selecting the coupler means comprises: polarizationindependent coupler means which comprises an input port, abi-directional port and an output port, and which is for conveying lightbetween the input port, the bi-directional port and the output port,with negligible polarization dependent loss of the light, such thatlight input at the input port becomes light output from thebidirectional port, light input at the bi-directional port becomes lightoutput from the output port, and the light is substantially unchanged asit passes in either direction between the bi-directional port of thepolarization selecting coupler means and the bi-directional port of thepolarization independent coupler means; polarization controller meansfor converting the state of polarization of light input at the inputport of the polarization selecting coupler means into a particularlaunch state of polarization of light input at the input port, of thepolarization independent coupler means, such that the particular launchstate of polarization is selected as specified by the launch controllermeans; and polarization parallel analyzer means for selecting two ormore particular receive states of polarization of the light output fromthe output port of the polarization independent coupler means to becomethe two or more separate channels of light output from the output portof the selecting coupler means.
 10. The apparatus according to claim 9in which the launch controller means specifies, and the polarizationcontroller means selects, only two particular launch states ofpolarization at each of the wavelengths of the tunable source meanscontrolled by the launch controller means.
 11. The apparatus accordingto claim 9 in which the launch controller means specifies, and thepolarization controller means selects, only one particular launch stateof polarization at each of the wavelengths of the tunable source meanscontrolled by the launch controller means.
 12. The apparatus accordingto claim 9 in which the polarization parallel analyzer means comprises abeam separator means for separating input light into two or moreseparate beams with negligible polarization dependent loss; and in whichthe apparatus includes tow or more single polarizer means for selectinga particular receive state of polarization for each of the said two ormore separate beams.
 13. The apparatus according to claim 12 in whichthere are three of the single polarizer means, in which a first one ofthe single polarizer means comprises a linear polarizer, in which asecond one of the single polarizer means also comprises a linearpolarizer but with a different orientation of its polarization axis, andin which a third one of the single polarizer means comprises a circularpolarizer.
 14. The apparatus according to claim 9 in which thepolarization parallel analyzer means selects only two particular receivestates of polarization of the light output from the output port of thepolarization independent coupler means for each of the particular launchstates of polarization light input at the input port of the polarizationindependent coupler means at each of the wavelengths of the tunablesource means, and in which the processor means is such that the state ofpolarization of the light backscattered by the optical fiber is deducedfrom the two particular receive states of polarization.
 15. Theapparatus according to claim 14 in which the polarization parallelanalyzer means comprises a beam separator means for separating inputlight into only two separate beams; and in which the apparatus includesonly two single polarizer means for selecting a particular receive stateof polarization for each of the said two separate beams.
 16. Theapparatus according to claim 15 in which a first one of the singlepolarizer means comprises a linear polarizer, and in which a second oneof the single polarizer means also comprises a linear polarizer but witha different orientation of its polarization axis.
 17. The apparatusaccording to claim 15 in which a first one of the single polarizer meanscomprises a linear polarizer, and in which a second one of the singlepolarizer means comprises a circular polarizer.